Making calcium metaphosphate fertilizer



Patented Sept. 26, 1939 MAKING CALCIUM METAPHOSPHATE FERTILIZER Harry A. -()urtis, Knoxville, Tenn.

No Drawing.

Application February 15, 1938,

Serial No. 190,625

4 Claims.

(Granted under the act of March amended April 30, 1928; 370 0.

The invention herein described may be manufactured and used by or for the Government for governmental purposes without me of any royalty thereon.

the payment to This invention relates to the process of making calcium metaphosphate, which is available as a plant food, particularly from rock phosphate and phosphoric acid. g

One of the objects of this invention is to provide a method for making a highly concentrated plant food by the use of dilute phosphoric acid,

such as produced by the wet process.

Another object of this invention is to provide a continuous and rapid process for the production of concentrated plant food. Still another object of this invention is to tion of a highly concentra the most part,

nomical means phate. which is more highly co provide a method for the producted fertilizer which, for w materials and equip- Other objects on of an ecoutilizes ra for converting monocalcium phosavailable as a plant food into a ncentrated plant food.

Hitherto it has been the general practice to treat rock phosphate with sulfuric acidand thereby produce a superphosphate containing'16 to 20% by weight A more recent in ph of P205 available as plant food. development, which has resulted the production of a small proportion of phosatic fertilizer in a to treat rock phosphate with phosphoric'acid and more concentrated form, is

thereby produce a double or triple superphosphate containing 40to 45 as plant food.

- is to treat rock oxide and thereb trated phosp latter process,

tion. It is, th

by weight of P205 available A, ph

y produce a still more concen hatic fertilizer containing 60 to 70% by weight of P205 available as plant food. This however, requires substantially different and expensive e quipment for its operaerefore, particularly desirable to utilize the general available rawm aterials and the maximum amou produce a more economical material, namely, a

more highly concentrated phosphatic fertilizer;

I have discovered a process of making calcium metaphosphate from a calcium which is available as a plant'food ontaining material; which "is phosphoric acid,

least partially a heated zone temperature gr still more recent development osphate with phosphorus pentnt of existing equipment to by permitting the mixture to at et, by feeding the mixture in'to j and by maintaining va sumcient adient in the heated zone succes-.

3, 1883, as G. 757) sively to remove the free water from the mixture and produce monocalcium phosphate, to convert the monocalcium phosphate into a calcium metaphosphate, which is only partially available as a plant food, and to fuse the calcium'metaphosphate making it available as a plant food.

It is evident that there are numerous factors which will influence conditions for the most satisfactory operation of my invention, the actual limits of which cannot be established except by a detailed study of each set of raw materials and the intermediate and finished products involved. The availability of the P205 in monocalciumphosphate is well, known, but several investigators have reported that calcium metaphospliate contained only a portion of its P205 in a form available as-plant food. It has been found that monocalcium phosphate may be dehydrated to calcium, metaphosphate under a variety of conditions with the general result that the P205 in the product. is, at most, only partly available as plant'food. This may be accomplished by heating at a single temperature, such as 300 C. or 400 C., foran extended period oftime, ;or by heating the original monocalcium phosphate through a constantly increasing temperature range of several hundred degrees centigrade. The calcium metaphosphate so produced, and containing itsv P205 in .an only partially avail-' able form, may be furtherheated through an increasing temperature range until it is fused, whereupon practically all of its P205 content becomes available, The changes in the availability of the P205 through the range from monocalcium phosphate to. fused calcium metaphosphate, in which .the monocalcium phosphate was'heated.

under uniform conditions at the intermediate temperatures, are indicated by the results re-- corded in the following table:

i v l a P305 Temperature, "0. 1 Citrate V Total soluble percent percent I oftotal 56:3 99.6 I 70.2 59, "7L0 55 71.6 48 o 71.0 33 71.2 30

process'imay be any material which is reactive The calcium containing material used inl'my Y Y with phosphoric acid to produce monocalcium phosphate. Ordinarily rock phosphate is preferred for this purpose but limestone, or any other readily available calcium containing material, may be equally suitable.

The phosphoric acid used in my process may be either a dilute or a concentrated acid. Commercially, the dilute acid containing 30 to 45% by weight of H3PO4 is produced by the wet process, that is, by the treatment of rock phosphate and sulfuric acid. The concentrated phosphoric acid, usually considered as containing at least 65% by weight of HaPO4, may be obtained by direct concentration of the dilute wet process acid or, as is more commonly the case, by electro-thermal methods.

The dilute phosphoric acid, or concentrated phosphoric acid, is mixed with the calcium containing material, preferably rock phosphate, to form a mixture which is to be charged to a heated zone. It is preferable to permit this initial mixture to stand a suihcient time for it to at least partially set. When, for example, concentrated acid is used, the few minutes time required for conveying the mixture from the mixer to the heating zone, on equipment such as a belt conveyer, is sufficient for the purpose. It is not necessary that this mixture be stored in the conventional manner to obtain the ultimately substantially complete conversion of the rock phosphate into monocalcium phosphate, although this procedure may be used if desired.

The mixture of rock phosphate and phosphoric acid is charged into a heated zone, at least a portion of which is capable of being heated to at least the melting point of the fused calcium metaphosphate subsequently produced. Pure calcium metaphosphate melts at 975 C., while the commercial product may melt below or above this temperature, depending upon the impurities present. However, the zone of highest temperature should be so controlled that the fused product withdrawn therefrom has attained a temperature of the order of 1000 to 1200 C. The'entire heated zone may be one which may be operated with a substantial temperature gradient between the entrance and exit of the zone, such as in the case of a rotary kiln. It is. sometimes necessary to alternately vary the location of the higher temperature zones slightly in the rotary kiln in order to remove ring-like masses of the calcium metaphosphate from-the temperature zone in which this material becomes highly plastic just prior to fusion. The heating may be carried out stepwise: for example, the initial portion of the heating carried out in a rotary kiln in which the maidmum temperature is below the fusion point of the calcium metaphosphate and the intermediate product so obtained then charged to a melting furnace in which the actual melting of the charge is obtained.

Regardless of the equipment in which it is found desirable to carry out the heating operation, the mixture of the calcium containing material and the phosphoric acid is heated under conditions such as successively to remove the free water from the mixture and produce monocalcium phosphate, to convert the monocalcium phosphate into calcium metaphosphate, which is only partially available as a plant food, and to fuse the calcium metaphosphate thereby making it available as a plant food.

It will be seen, therefore, that this invention actually may be carried out by the use of various modifications and changes without departing from its spirit and scope, with only such limitations placed thereon as are imposed by the prior art.

I claim:

1. Process of making calcium metaphosphate which is entirely and readily available as plant food from a calcium containing material, reactive with phosphoric acid to produce monocalcium phosphate, and phosphoric acid which comprises making a mixture of the fine calcium containing material and an aqueous solution of phosphoric acid containing 30 to 45% by weight of H3PO4, in substantially stoichiometrical proportion for the formation of monocalcium phosphate; feeding the mixture through a plurality of heated zones; maintaining the heated mixture at an increasing temperature gradient up to 1000 to 1200, C. adapted successively to remove the free water from the mixture and produce monocalcium phosphate, to convert the monocalcium phosphate into a calcium metaphosphate, which is only partially available as a plant food, and to fuse the calcium metaphosphate thereby making it entirely and readily available as a plant food and cooling the fused calcium metaphosphate to solidify the same and maintain it readily available as plant food.

2. Process of making calcium metaphosphate which is entirely and readily available as plant food from a calcium containing material, reactive with phosphoric acid to produce monocalcium phosphate, and phosphoric acid which comprises making a mixture of the fine calcium containing material and an aqueous solution of phosphoric acid, in substantially stoichiometrical proportion for the formation of monocalcium phosphate; feeding the mixture through a plurality of heated zones; maintaining the heated mixture at an increasing temperature gradient up to 1000 to 1200 C. adapted successively to remove the free water from the mixture and produce monocalcium phosphate, to convert the monocalcium phosphate into a calcium metaphosphate, which is only partially available as a plant food, and to fuse the calcium metaphosphate thereby making it entirely and readily available as a plant food and cooling the fused calcium metaphosphate to solidify the same and maintain it readily available as plant food. 3. Process of making calcium metaphosphate which is entirely and readily available as plant food from a calcium containing material, reactive with phosphoric acid to produce monocalcium phosphate, and phosphoric acid which comprises making a mixture of the fine calcium containing material and an aqueous solution of phosphoric acid, in substantially stoichiometrical proportion for the formation of monocalcium phosphate; feeding the mixture through a plurality of heated zones; maintaining the heated mixture at an increasing temperature gradient adapted successively to remove the free water from the mixture and produce monocalcium phosphate, to convert the monocalcium phosphate into a calcium metaphosphate, which is only partially available as a plant food, and to fuse the calcium metaphosphate thereby making it entirely and readily available as a plant food and cooling the fused calcium metaphosphate to solidify the same and maintain it readily available as plant food.

4. Process of making calcium metaphosphate which is entirely and readily available as plant food from a calcium containing material, reactive with phosphoric acid to produce monocalcium phosphate, and phosphoric acid which comprises cium metaphosphate, and to fuse the calcium metaphosphatc, thereby making it entirely and readily available as a plant food; and cooling said fused calcium metaphosphate to solidify the same and to maintain it readily available as a plant 5 food.

HARRY A. CURTIS. 

